Nov_Dec_AMP_Digital

iTSSe TSS A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 0 4 5 iTSSe TSS prominent advantage of these coatings was improved thermal cyclic life, along with excellent erosion resistance for critical components such as rotating airfoils. The major shortcomings for EB-PVD, which still hold true today, include the high capital cost for the equipment and the labor-intensive production process. Not being best-suited to the insulation design target, EB-PVD coatings also showhigher (> 20%) thermal conductivity values compared to legacy, po- rous air plasma-sprayedTBC coatingswith similar chemistry [3] . Due to the high cost and production-related challenges of EB- PVD coatings, the process was not considered favorable for land-based turbine applications. The necessity to overcome these challenges drove IGT manufacturers, along with their key coating applicators, to develop their own solutions. Proprietary air plasma spray pro- cesses and advanced coating microstructures were designed to support the specific application needs for the significantly larger components associated with IGT engines (e.g., transi- tion ducts, combustor liners, blades, and vanes). The lessons learned from the development and application of EB-PVD coatings, especially their columnar microstructures, set the stage for seeking new TBCs that also exhibit columnar-like features. Some of these newly developed microstructures, such as segmented or cracked, offered significant benefits over tra- ditional porous TBCs. The main benefits included improved thermal cyclic life and increased erosion resistance. Segment- edTBCsbecamemoreattractive, as theyalsooffered improved spray economics. In addition, the atmospheric plasma spray (APS) process enabled a path to coat larger IGT components with higher coating thicknesses that offset the higher conduc- tivity associated with dense segmented 7-8 wt% YSZ coatings compared to their porous counterparts. Further advancements onmicrostructure, targeting that of EB-PVD coatings, led to the evolution of suspension plasma spray (SPS) technology in the late 1990s, although its maturity is still yet to be achieved [4] . Vertical segmentation of TBCs has been developed in different forms (Fig. 2). The excellent service life achieved with EB-PVDmicrostructures iswhat inspired the designof newAPS TBCs to also exhibit some form of segmentation. The EB-PVD columnar microstructure is achieved by creating a buildup of the ceramic TBC material by growth from the gaseous phase (Fig. 2a). Columnar segmentation via the EB-PVDprocess is the preferredmethod of applying TBCs on the rotor blades of aero engines. The first air plasma-sprayed segmented microstructure that was developed is the vertically cracked segmentation known as dense vertically cracked (DVC) coatings. This type of segmentation is achieved via the APS process by applying a dense ceramic top coat using tightly controlled spray parame- ters and powders. This process typically utilizes high-enthalpy plasma conditions or requires the deposition of the ceramic on top of a 3D-textured surface, such as a cast or a weld-depos- ited grid, to produce the vertical cracks [2] . Crack-segmented coatings are now one of the standards used for components of industrial gas turbines [5] . Figure 2b shows a typical APS DVC microstructure deposited on a rough surface. In addition to the legacy DVC TBCs applied via APS, Fig. 2 — Different forms of vertical segmentation for thermal barrier coatings. FEATURE 9